1. Field of the Invention
This invention relates to a tandem brake roller system which prevents loads traversing the same from travelling over the conveyor too rapidly. More particularly, the invention relates to a braking system in which a series of tandem brakes apply a braking force to the bottom surface of the load.
2. Description of the Prior Art
Brake rollers are frequently employed in conveyor systems to control the speed of conveyed goods. In general, these devices utilize a special brake roller inserted among the plurality of rollers on the gravity conveyor. These rollers produce a braking force and control the maximum rate of advance of the load being conveyed down the gravity roller conveyor. These retarding rollers often were liquid filled as shown in U.S. Pat. Nos. 2,618,370 to Orwin, 3,511,350 to Stein, 2,607,459 to Ludwig et al. and 3,399,752 to Orwin. Non-liquid filled braking devices such as that shown in U.S. Pat. No. 3,655,021 to Froio, U.S. Pat. No. 3,918,561 to Isacsson and U.S. Pat. No. 4,275,572, to Holmes et al., have also been utilized.
Certain designs employ rotating brake rollers having internally fitted centrifugal weights which afford the desired braking action. These brakes are normally mounted in fixed position and directly contact the bottom surface of the conveyed goods.
Occasionally they are mounted with springs or other floating devices to provide traction or to accommodate surface irregularities of the conveyed goods. However, with a flexible mount there is only a single contact point when engaging the bottom surface. Another known method is to mount the braking roller indirectly, underneath one or two free rotating rollers which have axles firmly attached to the conveyor. Although this may provide two contact points, they are not flexible and do not accommodate surface irregularities.
One of the difficulties encountered in utilizing any of these prior art braking devices is that a braking action is developed only when the bottom surface of a load engages the braking roller. Frequently, load support bases are used, for example, pallets. A common type of pallet used in conveyor systems is the GMA (Grocery Manufacturers Association) pallet which has several bottom boards which run perpendicular to the direction of travel. When a conventional brake is used to control the speed of such pallets, it is mounted, either rigidly or by springs. As braking action is only achieved when the brake contacts the pallet bottom boards, there is a tendency for runaway conditions to occur due to the large spaces between the bottom boards.
When such a non-continuous surface moves across a braking roller, a braking action is applied only for the time of engagement between the roller engaging surface on the support surface and the braking roller. If the roller engagement support surfaces, for example, on a wooden pallet, is an eight inch wide board, braking action is developed only for the time during which the eight inch width of the board engages the braking roller. If the next board acting as a roller engagement support surface is spaced, for example, a foot or more behind the initial roller engagement support surface, then no braking action occurs until the second engagement surface again engages the braking roller. Additionally, an eight inch board which is uneven or damaged will have an engagement surface of less than eight inches, and even possibly may be so warped as to provide no engagement surface at all.
The net result of using such a system with the above described pallets is that the speed of the palletized load or packages travelling over the gravity roller conveyor is not properly controlled. This results in damage to the loads being transported or to the gravity roller conveyor itself. While it is possible to increase the number of braking rollers utilized in a given conveyor system this solution has been found to be very costly and complex. An example of this is disclosed in Newman, U.S. Pat. No. 4,383,598, which involved many brake units. However, this patent requires mechanical or electronic sensors and electronic circuitry which activates pneumatic cylinders.
Other prior art devices utilize a conveyor belt and a motor to provide retarding torque. In addition to consuming energy, these arrangements are complex and require frequent care.